Refactor: Use more specific type in argument of DB.Update (#162)

The DB.Update function takes a type of ACMETxt. However, the function
only requires the Value and Subdomain fields.

Refactor the function such that it takes ACMETxtPost instead of the full
ACMETxt record. This will simplify extraction of txt-record related
logic from the db code.

dns_test.go

@@ -168,7 +168,7 @@ func TestResolveTXT(t *testing.T) {
 		return
 	}
 	atxt.Value = validTXT
-	err = DB.Update(atxt)
+	err = DB.Update(atxt.ACMETxtPost)
 	if err != nil {
 		t.Errorf("Could not update db record: [%v]", err)
 		return